1. Cross-Reference to a Related Application
A related application entitled PRECURSORS FOR BORON NITRIDE COATINGS, U.S. Ser. No. 07/312,956, to Paine et al., filed Feb. 17, 1989 (now abandoned), the specification thereof is incorporated herein by reference.
2. Field of the Invention
This invention relates to monomer and polymer precursors useful for the formation of boron nitride articles and coatings.
3. Description of the Related Art
Non-oxide ceramic materials, such as borides, carbides and nitrides, in one or more crystalline modification, are known for their high mechanical strength, hardness, corrosion resistance, oxidation resistance and thermal shock stability. One commercially important example is provided by boron nitride, BN. Boron nitride has several crystalline modifications with the common hexagonal (.alpha.) form being isostructural with graphite and the cubic (.beta.) form being isostructural with diamond. Despite these structural similarities, boron nitride has much more favorable physical and chemical properties in comparison to carbon. For example, boron nitride has a high melting point (3000.degree. C.), high anisotropic thermal conductivity, excellent dielectric properties, low chemical reactivity and high temperature semiconductor characteristics. Applications for hexagonal boron nitride include crucibles for metal evaporation, transistor heat sinks, nuclear reactor control rods, high temperature (800.degree. C.) solid lubricants, metal corrosion resistant coatings and ceramic fiber coatings.
Boron nitride has been typically prepared in the prior art by high temperature pryolysis (900.degree.-1200.degree. C.) of simple boron and nitrogen containing materials, e.g. B(OH).sub.3 and urea. More recently, boron nitride has also been prepared by chemical vapor deposition (CVD) of mixtures of BCl.sub.3, BF.sub.3, or B.sub.2 H.sub.6 with NH.sub.3. In each case, the .alpha.-form of boron nitride is normally obtained, and it is most often produced as a powder. Increasing demands for fibers, films, foams, etc., require new approaches to obtain .alpha.- and .beta.-boron nitride.
There is currently a large effort underway in the United States and abroad in the utilization of polymer pyrolysis as a route to solid state materials. This concept, coupled with sol-gel and aerogel processing techniques, which were developed for silica glass processing, has allowed for the generation of new forms of well known solid state materials, as well as new families of materials. In particular, new oxide glasses, carbide fibers, silicon nitride and boron carbide have been obtained from pyrolysis of appropriate polymers. However, very little effort has been devoted to preparing boron nitride by polymer precursor techniques since very few characterized polymers containing boron and nitrogen have been reported. A brief outline of some pertinent work which is in published literature and/or in patents is provided below.
Several efforts were made to prepare polymeric boron-nitrogen containing compounds in the early 1950s through 1960s. A good deal of the work that was published involved borazine and substituted borazine as the monomer species. Harris, J. Org. Chem., 26, 2155 (1961), reported N-B coupling of two borazine, but no polymers were described. Laubengayer, et al., J. Am. Chem. Soc. 83, 1337 (1961), reported on the thermal decomposition of the parent borazine H.sub.3 B.sub.3 N.sub.3 H.sub.3. It was suggested that polymeric intermediates were formed, but they were not characterized. Wagner, et al., in "Synthetic High Polymers," Chemical Abstracts, 37349W, Vol. 66, p. 3685 (1967); "Borazine Polymers. B-N Linked Borazine Rings and Polyborazylene Oxides," Inorganic Chemistry, Vol. 1, pp. 99-106 (1962), and U.S. Pat. No. 3,288,726, entitled B-N LINKED BORAZENE DERIVATIVES AND THEIR PREPARATION, described the pyrolytic dehydrogenation of substituted borazine and resultant formation of N-B coupled borazine where the coupling chemistry directly linked B and N atoms in two rings. Wagner also described coupling of two borazine rings via an exo oxygen atom giving a B--O--B bridge. In the first case, it was proposed that ten rings could be coupled while, in the latter case, it was suggested that two to 23 rings could be coupled. In the ' 726 patent, a good deal of cross-linking chemistry involving substituted borazine was described. Horn, et al., in U.S. Pat. Nos. 3,345,396, entitled ORGANO-SUBSTITUTED BORAZINES; No. 3,392,181, entitled CYCLIC BN-COMPOUNDS; and No. 3,382,279, entitled PROCESS FOR THE PRODUCTION OF SILICON-CONTAINING N;N':N"-TRIORGANO-B:B':B"-TRIHYDRIDO-BORAZOLES; reported more complex polymerization chemistry involving large organic coupling agents. A. Meller, in Monatsh. Chem. 99, 1670 (1968), reported reactions of amino substituted borazine with diborane, which led to cleavage of the amino group on the borazine. No mention was made of the use of the polymers described in these reports for boron nitride precursors and no extensive high temperature pyrolysis chemistry was examined.
Patterson, in U.S. Pat. No. 3,321,337, entitled PROCESS FOR PREPARING BORON NITRIDE COATINGS, described an ambient pressure chemical vapor deposition process for .alpha.-boron nitride deposition on metals using B-trichloro borazine, Cl.sub.3 B.sub.3 N.sub.3 H.sub.3.
Taniguchi, in Japan Kokai 76 53,000 (Chem. Abstr. 85, 96582v (1976), reported the formation of filaments and films of boron nitride by extrusion and pyrolysis of a polymer obtained by heating (H.sub.2 NBNC.sub.6 H.sub.5).sub.3. No further details of the formation, characterization and processing of the polymer have appeared.
In 1978, Meller, et al., in Z. Naturforsch, 88b, 156-158, reported reactions of B-2-alkyl,-4,6-dichloro,N-1,3,5 trimethylborazines with hexamethyl disilizane, which produced polymers which were only partially characterized. Pyrolysis chemistry of the polymers was not described.
In 1984, Paciorek, et al., in Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 25(1), 15 (1984) (Abstr.), reported condensation reactions of several substituted borazine. All of this chemistry presumably involved direct ring-ring coupling (B-N bonds). Subsequently, the same group, Paciorek, et al., in Chemical Abstracts, 104, 211725v. (Abstr.); U.S. Pat. No. 4,581,468, entitled BORON NITRIDE PRECERAMIC POLYMERS; and "Boron-Nitrogen Polymers. I. Mechanistic Studies of Borazine Pyrolyses," Journal of Polymer Science, Vol. 24, pp. 173-185 (1986); discussed the use of these polymers as preceramic polymers, and they described some limited pyrolysis chemistry. They claimed that boron rich boron nitrides were obtained as black solids.
Bender, et al., in Cerm. Eng. Sci. Proc. 6, 1171 (1985), in collaboration with Paciorek, examined further details of the pyrolytic chemistry of substituted borazine including the monomer (H.sub.2 NBNC.sub.6 H.sub.5).sub.3 utilized by Taniguchi. In contrast to Taniguchi, they observed the formation of amorphous non-stoichiometric (boron-rich) materials from this precursor. Other borazine offered some promise for production of boron nitride fibers.
Numerous references to the conversion of .alpha.-boron nitride to .beta.-boron nitride have appeared and the vast majority depend upon the high temperature-high pressure recrystallization of a .alpha.-boron nitride prepared from classical thermal routes, e.g., pyrolysis of boric acid and urea. (See Moore, U.S. Pat. No. 3,578,403, entitled RECRYSTALIZATION OF PYROLYTIC BORON NITRIDE, to Zhdanovich, and U.S. Pat. No. 4,361,543, entitled PROCESS FOR PRODUCING POLYCRYSTALS OR CUBIC BORON NITRIDE, to Zhdanovich, et al.). In a different approach, Beale, in U.S. Pat. No. 4,655,893, entitled CUBIC BORON NITRIDE PREPARATION UTILIZING A BORON AND NITROGEN BEARING GAS, reported formation of .beta.-boron nitride by activated reactive evaporation of borazine (HNBH).sub.3 and a metal (Cr, Ni, Co, Al, Mu) catalyst.
In 1985, Hirano, et al., in U.S. Pat. No. 4,545,968, entitled METHODS FOR PREPARING CUBIC BORON NITRIDE SINTERED BODY AND CUBIC BORON NITRIDE, AND METHOD FOR PREPARING BORON NITRIDE FOR USE IN THE SAME, and U.S. Pat. No. 4,590,034, entitled METHOD FOR PREPARING SINTERED BODY CONTAINING CUBIC BORON NITRIDE AND METHOD FOR PREPARING CUBIC BORON NITRIDE, described very generalized high temperature--high pressure routes to .beta.-boron nitride involving borazine and substituted borazine.
Specific polymeric boron-nitrogen compounds have been recently developed as pyrolysis precursors to boron nitride. One such process uses the borazine, Cl.sub.3 B.sub.3 N.sub.3 H.sub.3, as the primary monomer as follows: ##STR1## wherein Et.sub.2 O represents diethyl ether, and n represents an integer. This polymer is then pyrolized to produce solid powder .alpha.-boron nitride, as follows: ##STR2## This powder cannot be successfully used for a coating requiring liquid properties. Prior art publications describing this work include: Narula, et al., "Synthesis of Boron Nitride Ceramics From Poly(borazinylamine) Precursors," Journal of American Chemical Society, (1987); "Precursors to Boron-Nitrogen Macromolecules and Ceramics," Mat. Res. Soc. Symp. Proc. Vol. 73, p. 383 (1986); and "New Precursors to Boron-Nitrogen Macromolecules and Ceramics," Abstr. H6.4 Mat. Res. Soc. Meeting, Spring 1986.
Paine, et al., in U.S. Pat. application U.S. Ser. No. 07/312,956, entitled PRECURSORS FOR BORON NITRIDE COATINGS filed Feb. 17, 1989 (now abandoned) describes the application of boron nitride coatings on various articles.